An active material is any material which responds electrically (e.g., produces a charge) to a mechanical stimulus (e.g., a mechanical stress or strain), or conversely which responds mechanically (e.g., produces a mechanical deformation) to an electrical stimulus (e.g., voltage, current or electric field). Active materials are also referred to as induced strain actuators and may be, for example, piezoelectric materials, electrostrictive materials, piezoresistive materials, shape memory materials or magnetostrictive materials. Thus, active materials, such as piezoelectric materials are well suited for use as sensors to detect mechanical disturbances, e.g., shocks, forces and pressures. Shock sensors have application in a number of industries, e.g., the automotive industry as sensors for air bag deployment, and the computer industry as sensors to detect shocks to disk drives and input pens.
According to their application, shock sensors are expected to detect various types of shocks. Generally, shocks can be decomposed into translational components and rotational components. A linear shock is defined as a shock having only translational components. A rotational shock is defined as a shock having only rotational components. Both linear and rotational shocks can be decomposed with respect to the three cartesian coordinate axes. Most shocks are a combination of linear and rotational shock.
Conventional shock sensors suffer from a number of drawbacks. Many conventional sensors only detect shocks along or about a number of axes equal to the number of sensors employed. For example, such a conventional shock sensor is disclosed in U.S. Pat. No. 5,521,772 issued to Lee et al. Lee et al. disclose a data disk drive including an acceleration rate sensor 50 for controlling or modifying the operation of the disk drive. Sensor 50 includes two piezoelectric transducers 52 and 54 separated by a stainless steel block 51. Block 51, transducers 52 and 54 and seismic mass plates 56 and 58 together form sensor 50. Sensor 50 is sensitive to shocks along only two axes. That is, sensor 50 is sensitive to linear shocks along the primary sensitive axis (i.e., the polling direction .beta.) of the two transducers (col. 6, lines 1-7) and to rotational shocks in the plane of the sensor (col. 5, lines 59-67). The response produced by sensor 50 is a result of mechanical stress acting on transducers 52 and 54 through flexion of sensor 50 about its sensitive axis.
U.S. Pat. No. 5,452,612 issued to Smith et al. discloses an accelerometer 10 having a beam-type transducer structure b 22. Transducer 22 includes a polarized piezoelectric sensor 59 and two sensing areas defined by output electrodes 57 and 58 (FIGS. 7-9). Transducer 22 is sensitive to shocks along two linear axes and one rotational axis (col. 6, lines 9-18). In order to achieve this three-axis sensitivity with two sensing areas, Smith et al. manufacture transducer 22 with an upward slant with respect to the plane in which the sensor lies in order to achieve sensitivity along an additional linear axis.
U.S. Pat. Nos. 5,235,472 and 5,373,213 issued to Smith disclose shock load detection devices 24 having a transducer subassembly 25 very similar to that disclosed by Smith et al. In addition to manufacturing transducer subassembly 25 with a particular orientation, Smith mounts detection device 24 in a particular orientation on the printed circuit board. This complex manufacturing and mounting scheme enables sensitivity to both torsional and linear forces in three mutually perpendicular directions (FIG. 3 and col. 4, lines 56-63).
Although the transducers disclosed by Smith et al. and Smith enable sensing along multiple axes, they have various drawbacks as compared to other piezoelectric sensing mechanisms. Specifically, the slanted beam-type structure is difficult to manufacture.